1. Field of the Invention
The invention relates to improved methods for drying fuel cells, particularly solid polymer electrolyte fuel cells, which methods can improve cell lifetime.
2. Description of the Related Art
Fuel cell systems are presently being developed for use as power supplies in a wide variety of applications. Fuel cells convert fuel and oxidant reactants to generate electric power and reaction products. They generally employ an electrolyte disposed between cathode and anode electrodes. A catalyst typically induces the desired electrochemical reactions at the electrodes. The presently preferred fuel cell type for portable and motive applications is the solid polymer electrolyte (SPE) fuel cell which comprises a solid polymer electrolyte and operates at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the cathode and anode. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain ionomer similar to that used for the solid polymer membrane electrolyte (e.g., Nafion®). The electrodes may also contain a porous, electrically conductive substrate that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Flow field plates for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell series stack.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The electrons travel through an external circuit providing useable power and then electrochemically react with protons and oxidant at the cathode catalyst to generate water reaction product. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to react with the oxidant and electrons at the cathode catalyst.
Because the ionic conductivity in typical SPE fuel cell electrolytes increases with hydration level, the fuel cell stacks are usually operated in such a way that the membrane electrolyte is as fully saturated with water as is possible without “flooding” the cells with liquid water (“flooding” refers to a situation where liquid water accumulates and hinders the flow and/or access of gases in the fuel cell). In this way, maximum power output can be provided during normal operation. However, if flooding should occur, a drying procedure may be employed (e.g., as disclosed in U.S. Pat. No. 6,103,409) to correct the problem. The detection of a flooding problem may be accomplished in various ways (e.g., as disclosed in U.S. Pat. No. 5,763,113 or U.S. Pat. No. 5,945,229).
In some fuel cell applications, the demand for power can essentially be continuous and thus the stack may rarely be shutdown (e.g., for maintenance). However, in many applications (e.g., as an automobile engine), a fuel cell stack may frequently be stopped and restarted with significant storage periods in between. During shutdown and storage at below freezing temperatures, a significant amount of liquid water may condense in the stack and freeze. The presence of ice inside can result in permanent damage to the stack. Even if such damage is avoided, the presence of ice can still hinder subsequent startup. Thus, various drying procedures may be employed to reduce the water content prior to shutting down the stack for storage (e.g., as disclosed in U.S. Pat. No. 6,479,177, U.S. Pat. No. 6,358,637, or US 2003/0186093). Alternatively, drying procedures may be employed on startup (e.g., as disclosed in US 2004/0033395 or JP 2003-151598).
Repeated shutdowns at below freezing temperatures have been found to adversely affect fuel cell performance. However, it has also been found that stack performance can be recovered via the use of an appropriate drying method. For instance, one such method is disclosed in US 2003/0180586 in which the drying is accomplished when the stack is not operating.
While drying procedures are frequently employed in fuel cells, such procedures may damage the fuel cell and hence limit its lifetime. Accordingly, improved drying procedures are therefore desired.